. Atolls:
An atoll is a continuous or broken circular shape of coral that has formed around a sunken oceanic volcano called a seamount. The water in the center of an atoll is called a lagoon. Atolls are also formed in the shape of an island or by a series of islets that are separated by channels that flow between the lagoon and the ocean. Atolls that are formed around an island can act as a protective barrier for the island.
After a volcano breaks the water’s surface to create an oceanic island, the formation of an atoll begins. Hermatypic corals are living creatures that settle and form a hard fringing reef, made of calcium carbonate, along the sides of the volcano just below the water line. The hermatypic corals are found only in warm water, primarily in the Pacific and Indian oceans. The coral acts as a host for the algae and together they are co-dependent for survival. Light is necessary for
the algae to grow; therefore, the algae continually grows upward towards the photic zone of the ocean for its nutrients, building upon layers of dead coral.
The reef, also known as an exoskeleton, gradually separates from the volcanic island in the process called subsidence. The separation of the fringing reef from the oceanic island creates a shallow body of water called a lagoon. Charles Darwin was one of the earliest scientists to provide scientific insight into the origins of atoll development through his work on the, “subsidence theory of coral reef formation.”
Subsidence of the volcanic island occurs over a period of millions of years. Once the volcano stops erupting and spewing lava, it becomes cold and begins the reverse process of erosion. A flat top forms as the top of the volcano becomes susceptible to the ocean’s surf and wind. The flat top formation is called a guyot. The fringing reef becomes a barrier reef the further the seamount erodes into the sea creating a deeper lagoon that is further away from the shoreline. The barrier reef protects the inner lagoon area from the elements of the open ocean.
The continued subsistence of the guyot changes the dynamics of the surrounding reef. The outside, ocean facing, portion of the barrier reef will remain healthy and continue to grow because nutrients are plentiful creating a unique ecosystem for marine life. The inside portion of the reef that faces towards the lagoon begins to die. Competition for limited nutrients in the lagoon area prevents the algae from flourishing and ultimately the decay of the coral on this side of the reef. The decaying limestone reef affects the lagoon’s chemical composition which changes the color of the water from a deep blue to a bright teal.
Energy from the ocean’s waves and the elements work to erode and break apart the reef to create sand. The ocean’s organic and other miscellaneous materials build up on the reef to eventually form a distinctive ring-shaped island or islets called an atoll. In addition to erosion of an atoll, an atoll is also susceptible to destruction from the rising oceans due to the forces of global warming.
An atoll is historically hazardous to ships of all designs due to the atoll’s low-lying elevation. Most atolls are in remote locations that are uninhabited. These remote locations were used as nuclear bomb testing sites, including the first hydrogen bomb test. However, some atolls are inhabited and are popular tourist destinations because of their sandy beaches, calm lagoons and popular diving sites.
Evolution of an atoll video:
Atolls:
An atoll is a continuous or broken circular shape of coral that has formed around a sunken oceanic volcano called a seamount. The water in the center of an atoll is called a lagoon. Atolls are also formed in the shape of an island or by a series of islets that are separated by channels that flow between the lagoon and the ocean. Atolls that are formed around an island can act as a protective barrier for the island.
After a volcano breaks the water’s surface to create an oceanic island, the formation of an atoll begins. Hermatypic corals are living creatures that settle and form a hard fringing reef, made of calcium carbonate, along the sides of the volcano just below the water line. The hermatypic corals are found only in warm water, primarily in the Pacific and Indian oceans. The coral acts as a host for the algae and together they are co-dependent for survival. Light is necessary for
the algae to grow; therefore, the algae continually grows upward towards the photic zone of the ocean for its nutrients, building upon layers of dead coral.
The reef, also known as an exoskeleton, gradually separates from the volcanic island in the process called subsidence. The separation of the fringing reef from the oceanic island creates a shallow body of water called a lagoon. Charles Darwin was one of the earliest scientists to provide scientific insight into the origins of atoll development through his work on the, “subsidence theory of coral reef formation.”
Subsidence of the volcanic island occurs over a period of millions of years. Once the volcano stops erupting and spewing lava, it becomes cold and begins the reverse process of erosion. A flat top forms as the top of the volcano becomes susceptible to the ocean’s surf and wind. The flat top formation is called a guyot. The fringing reef becomes a barrier reef the further the seamount erodes into the sea creating a deeper lagoon that is further away from the shoreline. The barrier reef protects the inner lagoon area from the elements of the open ocean.
The continued subsistence of the guyot changes the dynamics of the surrounding reef. The outside, ocean facing, portion of the barrier reef will remain healthy and continue to grow because nutrients are plentiful creating a unique ecosystem for marine life. The inside portion of the reef that faces towards the lagoon begins to die. Competition for limited nutrients in the lagoon area prevents the algae from flourishing and ultimately the decay of the coral on this side of the reef. The decaying limestone reef affects the lagoon’s chemical composition which changes the color of the water from a deep blue to a bright teal.
Energy from the ocean’s waves and the elements work to erode and break apart the reef to create sand. The ocean’s organic and other miscellaneous materials build up on the reef to eventually form a distinctive ring-shaped island or islets called an atoll. In addition to erosion of an atoll, an atoll is also susceptible to destruction from the rising oceans due to the forces of global warming.
An atoll is historically hazardous to ships of all designs due to the atoll’s low-lying elevation. Most atolls are in remote locations that are uninhabited. These remote locations were used as nuclear bomb testing sites, including the first hydrogen bomb test. However, some atolls are inhabited and are popular tourist destinations because of their sandy beaches, calm lagoons and popular diving sites.
Evolution of an atoll video:
Darcee Heffner
Works Cited:
Evers, J. (Ed.). (1989,1993). Encyclopedic Entry, Atolls. Retrieved from National Geographic:
http://education.nationalgeographic.com/education/encyclopedia/atoll/?ar_a=1
http://www.youtube.com/watch?feature=player_detailpage&v=vthTvzjXTPE
JWalker. Age footstock. Bing fotos: Atolls of the Maldives. http://bingfotos.blogspot.com/search?q=atolls
McGinley, M. (2012). Atoll. Retrieved from http://www.eoearth.org/view/article/150305
Darcee Heffner